The invention relates to an image forming apparatus such as a copying machine, printer, and the like and particularly, to an image forming apparatus which uses a belt fixing unit.
An electrophotographic image forming apparatus forms a toner image on an image carrier, transfers the toner image to a transfer material by a transfer unit, fixes the image on the transfer material at a nip portion which is formed by a heating member and a pressing member in a fixing unit, and delivers the fixed transfer material to the outside of the apparatus.
Recently, image forming apparatus have been requested to process color images at high speed. To meet such a request, the fixing unit in the image forming apparatus must widen the width of the nip portion to take time for transfer materials to pass through the nip portion.
When the fixing unit is of a roller type which uses fixing rollers and pressure rollers, the diameters of the rollers must be greater to prolong the time during which transfer materials pass through the nip portion. However, this will enlarge the fixing unit and consequently make the image forming apparatus greater.
If the elastic surface layer of the fixing roller is made thicker to ensure the width of the nip portion, the rate of heat transfer from the heat source to the surface layer of the fixing roller will be reduced and the warm-up time will become longer. As the result, this will reduce the printing speed and will not be fitted for high-speed printing.
To solve such a problem in the roller type fixing unit, a fixing belt type fixing unit has been proposed which presses a heated fixing belt against a pressure roller to assure a great nip width.
The image forming apparatus has been also requested to satisfy the other demands such as two-sided printing of transfer materials in addition to high-speed color image printing.
In two-sided printing, when unfixed toner on the first face is fixed, the transfer material has a force to be attached to or wound on the fixing belt. When the transfer material is inverted upside down and unfixed toner on the second face is fixed, the transfer material concurrently has a force to be attached to the pressure roller due to the toner on the first face. The direction of this attaching force to or winding on the fixing belt or to the pressure roller depends on the print density of two faces and quantity of toner adhered to two faces.
As explained above, since the direction of the attaching or winding force of the transfer material to the fixing belt or to the pressure roller depends on the print state of the first and second faces of the transfer material, sheet separation claws are provided in contact with the fixing belt and the pressure roller to prevent the transfer material from attaching thereto.
However, the separation claws may possibly cause damages on members with which the claws are in contact and unevenness in image gloss on the image obtained. The following technology (for example, Patent Document 1) without separation claw has been proposed to solve such problems, that is, to perform two-sided printing without unwanted sheet attaching to the fixing belt and the pressure roller.
That is, a belt-type fixing unit has an endless fixing belt which is entrained about a support roller and a heating roller, and a pressure roller which is provided opposite to the support roller through the fixing belt to press against the support roller. A pressure nip portion is formed between the pressure roller and the fixing belt, and a transfer material with an unfixed toner image is conveyed to pass through this nip portion for fixing. This technology is characterized in that the section of the nip portion shows an approximate straight line to prevent a two-sided copying sheet from attaching to the belt and the pressure roller. [Patent Document 1]: Japanese Unexamined Patent Publication 2004-226815
However, since the technology disclosed by Patent Document 1 limits the sectional shape of the pressure nip portion to an almost straight line shape, this configuration cannot solve the sheet attaching problem if the printed faces have different print densities thereon and toner quantities adhered thereto.
Further, materials, shapes, and other conditions of the fixing belt, the support roller, and the pressure roller must be limited to some extent to make the cross-section of the pressure nip portion almost linear. This may reduce the degrees of freedom of designing of the fixing unit.